Molding Process—The type of molding process (such as green sand, shell, investment, etc.) has the greatest single influence on tolerance capability. How a given molding process is mechanized and the sophistication of its pattern or die equipment can refine or coarsen its base tolerance capability.

Casting Weight and Longest Dimension—Logically, heavier castings with longer overall dimensions require more tolerance. These two parameters have been defined statistically in tolerance tables for some alloy families.

Mold Degrees of Freedom—This parameter is least understood. Just as some molding processes have more mold components (mold halves, cores, loose pieces, chills, etc.) than others, some casting designs require more mold components. Each mold component has its own tolerances, and tolerances are stacked as the mold is assembled. More mold components mean more degrees of freedom; hence more tolerance. Good design for tolerance capability minimizes degrees of freedom in the mold for features with critical dimensions.

Draft—It is common for casting designs to ignore the certainty of draft, including mold draft, draft on wax and/or styrofoam patterns made from dies, and core draft. Since 1 degree of draft angle generates 0.017 in. of offset per in. of draw (about 0.5 mm/30 mm), draft can quickly use up all of a tolerance zone and more.

Patternmaker’s Contraction—The uncertainty of patternmaker’s contraction is why metalcasters normally recommend producing first article and production process verification castings (sometimes called “sample” or “capability” castings) to establish what the dimensions really will be before going into production. A common consequence of patternmaker’s contraction uncertainty is a casting dimension that is out of tolerance, not because it varies too much, but because its average value is too far from nominal. In other words, the dimension contracted more or less than expected.

Cleaning and Heat Treating—Many casting dimensions are touched by downstream processing. At the least, most castings are touched by abrasive cutting wheels and grinding—even precision castings. Many castings are heat-treated, which can affect straightness and flatness.

When considering the breadth and depth of geometry’s importance in casting design, from its influence on castability, the geometry of gating/risering, structural form, cosmetic appearances and downstream fixturing, extensive brainstorming of geometry is highly recommended. The standard for “optimal” casting geometry is high, but the possibilities for geometry are limitless. Find ways of exploring geometry quickly, such as engineering sketching, before committing to a print or solid model.